Valve actuation in an internal combustion engine is required in order for the engine to produce positive power. During positive power, one or more intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves also may be opened during positive power at various times to recirculate gases for improved emissions.
Engine valve actuation also may be used to produce engine braking and exhaust gas recirculation (EGR) when the engine is not being used to produce positive power. During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
In many internal combustion engines, the intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes that are an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle driveablity may be obtained if the intake and exhaust valve timing and lift can be varied. The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift in order to optimize them for various engine operating conditions, such as different engine speeds.
One proposed method of adjusting valve timing and lift, given a fixed cam profile, has been to provide variable valve actuation by incorporating a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid trigger valve in communication with the hydraulic circuit that includes the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit when the master piston is acted on by certain of the cam lobes. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the hydraulic fluid displaced by the motion of the master piston, which reciprocates in response to the cam lobe acting on it. When the solenoid is opened, the circuit may drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston and the engine valve.
Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system, although the aforementioned '841 patent does contemplate the use of a high speed trigger valve. High speed lost motion systems in particular, are needed to provide Variable Valve Actuation (VVA). True variable valve actuation is contemplated as being sufficiently fast as to allow the lost motion system to assume more than one length within the duration of a single cam lobe motion, or at least during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, sufficiently precise control may be attained over valve actuation to enable more optimal valve actuation over a range of engine operating conditions. While many devices have been suggested for realizing various degrees of flexibility in valve timing and lift, lost motion hydraulic variable valve actuation is becoming recognized for superior potential in achieving the best mix of flexibility, low power consumption, and reliability.
Engine benefits from lost motion VVA systems can be achieved by creating complex cam profiles with extra lobes or bumps to provide auxiliary valve lifts in addition to the conventional main intake and exhaust events. Many unique modes of engine valve actuation may be produced by a VVA system that includes multi-lobed cams. For example, an intake cam profile may include an additional lobe for EGR prior to the main intake lobe, and/or an exhaust cam profile may include an additional lobe for EGR after the main exhaust lobe. Other auxiliary lobes for cylinder charging, and/or compression release may also be included on the cams. The lost motion VVA system may be used to selectively cancel or activate any or all combinations of valve lifts possible from the assortment of lobes provided on the intake and exhaust cams. As a result, significant improvements may be made to both positive power and engine braking operation of the engine.
The foregoing benefits are not necessarily limited to exhaust and intake valves. It is also contemplated by the present inventors that lost motion VVA may be applied to an auxiliary engine valve that is dedicated to some purpose other than intake or exhaust, such as for example engine braking or EGR. By providing an auxiliary engine valve cam with all of the possible actuations that may be desired and a lost motion VVA system, the actuation of the auxiliary valve may be varied for optimization at different engine speeds and conditions.
In view of the foregoing, the lost motion system and method embodiments of the present invention may be particularly useful in engines requiring variable valve actuation for positive power, engine braking valve events (such as, for example, compression release braking), and exhaust gas recirculation valve events.
Each of the foregoing types of valve events (main intake, main exhaust, engine braking, and exhaust gas recirculation) occur as a result of an engine valve being pushed into an engine cylinder to allow the flow of gases to and from the cylinder. Each event inherently has a starting (opening) time and an ending (closing) time, which collectively define the duration of the event. The starting and ending times may be marked relative to the position of the engine (usually the crankshaft position) at the occurrence of each. These valve events also inherently include a point at which the engine valve reaches its maximum extension into the engine cylinder, which is commonly referred to as the valve lift. Thus, each valve event can be defined, at least at a basic level, by its starting and ending time, and the valve lift.
If the lost motion system connecting the engine cam to the engine valve has a fixed length each time a particular lobe acts on the system, then the starting and ending times and the lift for each event marked by that lobe will be fixed. Furthermore, a lost motion system that has a fixed length over the duration of the entire cam revolution will produce a valve event in response to each lobe on the cam, assuming that the system does not incorporate a lash space between the lost motion system and the engine valve. The optimal starting time, ending time, and lift of an engine valve is not “fixed,” however, but may differ widely for different engine operating modes (e.g., different engine load, fueling, cylinder cut-out, etc.), for different engine speeds, and for different environmental conditions. Accordingly, it is desirable to have a lost motion system that is not fixed in length, but rather “variable” over the short run, where the short run is as brief as the duration of time it takes for a cam lobe to pass a fixed point (i.e. as little as a few cam shaft rotation degrees), or at least no longer than one cam shaft revolution.
It is also desirable to provide optimal power and fuel efficiency during positive power operation of an engine. One advantage of various embodiments of the present invention is that they may be used to vary the intake and exhaust valve timing and/or lift to provide optimal power and fuel efficiency, if so desired. The use of a lost motion VVA system allows valve timing and/or lift to be varied in response to changing engine conditions, load and speed. These variations may be made in response to real-time sensing of engine conditions and/or pre-programmed instructions.
It is also desirable to reduce NOx and/or other polluting emissions from the exhaust of internal combustion engines, and diesel engines in particular. One advantage of various embodiments of the present invention is that they may be used to reduce NOx and other polluting emissions by carrying out internal exhaust gas recirculation or trapping residual exhaust gas using variable valve timing and auxiliary lifts of intake, exhaust, and/or auxiliary valves. By allowing exhaust gas to dilute the incoming fresh air charge from the intake manifold, lower peak combustion temperatures may be achieved without large increases in fuel consumption, which may result in less formation of pollution and more complete burning of hydrocarbons.
Also of great interest for diesel engines is the capability of the engine to have an engine braking mode. It is another advantage of various embodiments of the present invention to optimize engine braking across an engine speed range, as well as modulate engine braking responsive to driver demand.
It is also desirable to provide engines with the ability to warm up faster by employing special valve timing during a brief period after the engine is started. Driver comfort and after-treatment device efficiencies may depend on how quickly an engine can be brought up to normal operating temperature. Yet another advantage of various embodiments of the present invention is that they may provide improved engine warm up. This can be achieved using a number of different techniques, including, but not limited to, early intake valve closing, EGR, changes in exhaust/intake valve overlap, cylinder cut-out of some cylinders, and even compression release braking of some cylinders during positive power to effectively make the engine work against itself.
The ability to provide cylinder cut-out may be useful not only during engine warm-up and not only for diesel engines. In some embodiments of the present invention, the lost motion VVA system may be adapted to lose all cam motions associated with an engine valve or even an engine cylinder. As a result, these lost motion VVA systems may be used to effectively “cut-out” or shut off one or more engine cylinders from inclusion in the engine. This ability may be used to vary the number of cylinders that fire during positive power, to add control over fuel efficiency and power availability. Cylinder cut-out may also increase exhaust gas temperature in the cylinders that continue to fire, thereby improving the efficiency of exhaust after-treatment. It is also contemplated that cylinder cut-out could be carried out sequentially at the time an engine is turned on and/or off to decrease the amount of out of balance shake that is produced by an engine during start-up and shut-down periods.
However, having a hydraulic circuit with various valves transferring motion from a cam or other motion imparting device to an engine valve may possess an increased risk of valve or engine damage or engine failure in the event a solenoid or trigger valve fails. In such a failure situation, the VVA system may be disabled such that the engine valves associated with the VVA system do not open or close as desired. This may result in engine failure. Further, in a failure situation the VVA system may be disabled with an engine valve in an open position. Such a position may lead to valve or engine damage due to valve-piston contact. Thus, a VVA system with a fail-safe attribute may be desirable.
Further, a hydraulic circuit for transferring motion from a cam or other motion imparting device to an engine valve may cause problems for engine valve actuation during start-up and warm-up. This is because hydraulic fluid may drain from the hydraulic circuit as the engine sits in a state of non-use. When the engine is started, the hydraulic circuit between the cam and engine valves may be empty, and therefore the valve may not be actuated. Thus, a secondary method of actuating the engine valves during start-up or warm-up is desirable.
Space and weight considerations are also of considerable concern to engine manufacturers. Accordingly it is desirable to reduce the size and weight of the engine subsystems responsible for valve actuation. Some embodiments of the present invention are directed towards meeting these needs by providing a compact master-slave piston housing for the lost motion VVA system. Applicants have discovered that some unexpected advantages may also be realized by reducing the size of the lost motion VVA system. As a result of reduction of the overall size of the system, the attendant hydraulic passages therein may be reduced in volume, thus improving hydraulic compliance.
Additional advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.